The invention relates to magnetic actuators and magnetic position sensors. More particularly, the invention relates to magnetic bearings and magnetic reluctance motors, especially those employing feedback control systems.
Magnetic actuators, in the form of magnetic bearings, reluctance motors, and in other forms, have been known for some time. Separately, position sensors have also been known, and in fact have been used in conjunction with magnetic actuators.
U.S. Pat. No. 4,092,867 (Matzuk) discloses, among other things, an ultrasonic scanning apparatus in which an ultrasonic transducer is moved in a predetermined path by magnetic force-generating means. Preferably, servo-control is provided to control the movement of the transducer. The servo-control utilizes a position sensor to determine the position of the transducer in its path. The position signal is compared with a reference signal, representative of the desired position, in order to generate an error signal which is amplified and conditioned to provide a correcting signal which attempts to make the position exactly track the reference.
Particular magnetic force-generating (drive) means disclosed by Matzuk are shown in FIGS. 1-5 and 10-14. In each of these drive means, the driving force is generated by an interaction between the magnetic field of one or more permament magnets and the magnetic field of a current passing through one or more coils.
Position sensors are shown in FIGS. 7, 8, 9, 9a, 12, and 15 of Matzuk. One type of position sensor utilizes a vane element attached to the transducer substrate, a light source, and a light detector. The vane moves between the light source and the light detector, so that the amount of light detected represents the position of the transducer. Another position sensor (FIGS. 9 and 9a) utilize eddy-current vanes attached to the transducer substrate, and coils fixed to the housing, the coils surrounding the vanes. The changing inductance of the coils, due to varying eddy-current losses, indicates the transducer position.
Another position sensor disclosed by Matzuk (FIGS. 12 and 15) utilizes a vane attached to a swinging column. (The transducer is on the free end of the column.) A coil, fixed to the housing, is energized so that the changing inductance of the coil represents the column position and hence also the transducer position.
French Pat. No. 1,218,976 discloses a magnetic position indicator which measures the variation in the inductance of a coil to determine the position of a pilot blade P. A ferrite pole piece is provided with windings and an air gap. It is made part of an LC tuned circuit which is tuned to resonance at an applied frequency. When the blade P is moved into the air gap, the inductance of the coil increases, thereby changing the resonant frequency of the circuit.
The known devices suffer from several disadvantages. First, the separate actuator and position sensor occupy a relatively large volume. With the trend toward miniaturization, this is a problem. It becomes more of a problem when the actuator and sensor are used in, for example, a magnetic bearing on a satellite. Volume and weight are at a premium on satellites.
Second, the actuator and the position sensor present a relatively complex arrangement when combined. In some cases, where both the actuator and the sensor utilize magnetic fields, the arrangement must assure minimal interference between these fields. This may require magnetic shielding.
Third, in magnetic bearings in cryogenic coolers, the actuator and the sensor must be hermetically sealed to the pressure vessel. Separate actuators and sensors therefore require separate hermetic seals. The more hermetic seals used, the higher the cost and the greater likelihood that a failure will occur.
Finally, in some position sensors, notably eddy current sensors, the output signal is strongly temperature-dependent. This can be a problem, especially when the device is used in a harsh environment.